Tuesday, March 26, 2013

In this era of human domination of the world, a successful
animal or plant may be defined as one that has adapted in some way to human
presence, even benefits by it.

Cliff Swallows (Petrochelidon
pyrrhonota) may be one of those animals. They build mud nests that they
attach to vertical substrates overhung sufficiently to provide shelter from
rain. They were able to evolve this nesting habit because of the widespread
abundance of rocky canyons in western North America. Cliff Swallows are still
abundant in that habitat.

The species was probably restricted to western canyonlands
until barns and bridges built by settlers advancing across the Great Plains a
few hundred years ago provided nesting substrates comparable to those provided
by nature. The birds took advantage of these newly furnished nest sites and
moved eastward, encountering more and more artificial cliffs and canyons as
they went. Today they nest through much of the East as well, on buildings but mostly
under big bridges across rivers.

Cliff Swallows are among the most social of land birds, with
colonies of up to 3,500 pairs reported. Their closely packed nests extend over
lengthy sections of cliff walls and can fill up the sides of barns and bridges
wherever there is overhead shelter. Because they nest so densely, they are more
subject than most birds to ectoparasites, especially swallow bugs and ticks, in
their nests. Because of high chick mortality from high densities of these
parasites, whole colonies are sometimes abandoned and the birds settle
elsewhere.

Their other "enemies" include House Sparrows,
which take over Cliff Swallow nests and even kill their young, and automobiles,
which take their toll of birds nesting adjacent to roads. Recent research has
shown that Cliff Swallows in such situations are evolving shorter wings, making
them more maneuverable and less likely to be struck down by a car.

Swallow nests are somewhat messy, because after about a week
old, the young defecate off the edge of the nest. A pile of poop can build up
rapidly below the nest, sometimes even blocking the entrance! Swallows that
nest where people live and work aren't always well loved because of this, and
whole colonies are sometimes removed from highway overpasses because of the mess
they make. Living around humans brings mixed benefits.

Thursday, March 21, 2013

For some reason, purely by accident, it turns out that many
of the world's swallow species now nest primarily in structures built by
humans.

One of these species is the Violet-green Swallow (Tachycineta thalassina). Its breeding distribution
lies mostly in extensive mountain ranges of the West. Originally the birds must
have nested in natural crevices in trees and cliffs as well as holes excavated
by woodpeckers, chickadees or nuthatches. Some of the swallows still nest in
such places, and they remain common as montane birds.

However, fast forward to the early Twenty-first Century, and
one of the best places to find Violet-greens is in our cities and suburbs. When
I first moved to Seattle in 1968, there were nesting swallows all over the
city. Some of them nested in houses, even more in commercial buildings, for
example all over the University District. As I drove and walked around, I saw
them all over the place. Any little opening into a building often had a pair of
these beautiful birds nesting in it.

Nowadays they are still present in Seattle, but in reduced
numbers. I have heard of no place where they are increasing and many
neighborhoods from which they have disappeared. When I moved into a house in
Maple Leaf, a wooded section of town, in 1991, I could dependably see and hear Violet-greens
overhead on a daily basis each summer. A decade later, they were scarcely to be
seen, and by 2010, they had disappeared from former haunts in many parts of the
city.

A possible cause of this decline is the tidying up of our
human habitat. The crevices in buildings that are used by Violet-greens and
some other urban/suburban birds are presumably decreasing in number as people
find them and seal them up. Who could argue with someone who wants to keep rats
and mice out of their house!

Perhaps more significant, all swallows are aerial
insectivores. In other words, their diet consists almost entirely of flying
insects. Because of habitat destruction and pollution, primarily the use of
insecticides on so many of our crops, we have brought about a widespread
decline in such insects. Violet-green Swallows are decreasing generally as
breeding birds in the Pacific states, although Rocky Mountain populations are
doing well.

Even with local declines, swallows are thankfully still among
our most common and visible birds.

Tuesday, March 12, 2013

This isn't Capistrano, but our swallows are on their way
back from their wintering grounds. The first to come are Tree Swallows (Tachycineta bicolor), which winter in
southern US and Mexico. They probably winter the farthest north because they
are programmed to migrate so early. In fact, a very few birds sometimes winter
as far north as Ridgefield National Wildlife Refuge in southwestern Washington.

Tree Swallows are cavity nesters. But they can't excavate
their own cavities like a woodpecker can, so they depend on natural cavities,
old woodpecker holes, and nest boxes that we put up for them. They like to nest
over water when they can, as predators that can climb trees to prey upon
swallow eggs and young don't inhabit flooded wetlands.

Thus nesting cavities for them are quite limited, and there
is fierce competition for them among swallow pairs. The earlier a male swallow
returns to its breeding grounds, the more likely it will find an unoccupied
cavity. Once a cavity is occupied, it's an uphill battle for the next bird that
shows up to try to take it over, and so first come, first served.

Thus our Tree Swallows start trickling back to western
Washington by late February, and the trickle becomes a river by the beginning
of April. By that time, pretty much all available cavities are taken. Of course
many of these birds continue on north, all the way to Alaska.

This surprisingly early arrival, well before our real
spring, comes with a downside. The weather can be wretched at this time of
year, cold and windy and rainy. Swallows are aerial insectivores, and their
prey can be very hard to come by under such conditions. There is no doubt that
in an especially bad spring, some of the birds succumb to starvation. There is
no point in trying to raise a brood of young until conditions get better, so
they don't attempt that until considerably later, some time in April or May.

Male and female Tree Swallows look the same, beautifully
iridescent blue above and snow-white below, but with an interesting caveat.
First-year males look just like older birds, but first-year females are
recognizably different, dull brown on the back. There are scarcely any birds in
the World with a unique first-year female, but the Tree Swallow is one. The
adaptive significance of this first-year plumage is poorly understood.

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About the Slater Museum

Our goals are to preserve and provide a collection of specimens to be used for research and education. Located in Tacoma, Washington, the Slater Museum is one of the region's significant repositories for bird, mammal, reptile, amphibian and plant specimens from the Pacific Northwest. In addition, we strive to educate the local community about nature and about the value of museums.